This invention relates generally to catalysts for purification of liquid media.
Contamination of liquids, including water, with organic compounds is of great concern. Organic compounds contaminate liquids by various methods, including accidental discharge of organic compounds into water streams, leaks in vessels transporting organic compounds such as oil, and other ways. Water contaminated with organic compounds has direct and secondary effects on health and the environment.
Numerous processes have been developed for the purification of wastewater. Wet air oxidation (WAO) systems employ relatively severe operating temperatures (up to 350° C.) and high pressures (up to about 20 MPa) to decompose high concentrations of organic materials. In a typical WAO system, waste is pumped through a heat exchanger into a reactor along with compressed air. The oxidized content of the reactor is passed to a separator that partitions the gas and liquid fractions. The high temperature and pressure in the reactor maintains water in the liquid state and increases the solubility of oxygen supplied by the compressed air. These conditions result in a highly oxidizing environment that can be used to decontaminate a wide variety of industrial waste streams, such as those generated from pulp and paper mills, breweries, and chemical processing plants. One of the major uses of WAO is for treatment of sewage sludge, where the sludge is either completely oxidized or rendered suitable for disposal or biological treatment.
Through the use of appropriate catalysts in WAO systems, the severity of the processing conditions can be reduced to improve the economics of the operation. Moreover, non-catalyzed WAO produces lower molecular weight compounds as major byproducts of incomplete oxidation (Mishra, V. S. et al., Ind. Eng. Chem. Res. (1995) 32, 2; Leitenburg, et al., Appl. Catal. B: Environ. (1996) 11, L29-L35; Luck, F. Catal. Today (1999) 53, 81-91). These lower molecular weight compounds typically are carboxylic acids, alcohols, and aldehydes that are resistant to decomposition and can dictate the kinetics and resulting cost of the process. However, catalytic wet-air oxidation (CWAO) leads to complete oxidation of organic material to CO2 and H2O, thereby minimizing post-treatment processing of the contaminated water.
Both homogeneous and heterogeneous catalysts have been applied to WAO with some success; however, homogeneous catalysts are less desirable because they are difficult to remove from the treated effluent, which adds process steps and expense. Binary and ternary heterogenous catalysts based on Co/Bi, Cu/Co, Cu/Co/Bi, Ru/Ce, Mn/Ce (1:1), and Ru/Mn/Ce have been tested for acetic acid destruction with varying degrees of success (Mishra, V. S., et al. Ind. Eng. Chem. Res. (1995) 34, 2-48).
Imamura et al. compared the activity of 5% Pt, Ru, or Rh supported on CeO2 for destruction of poly(ethylene glycol) at 473 K and reported that Ru/CeO2 was most active (Imamura, et al. Ind. Eng. Chem. Res. (1988) 27, 718).
de Leitenburg et al tested CeO2, 80% CeO2-20% ZrO2, 76% CeO2-19% ZrO2-5% CuO, 76% CeO2-19% ZrO2-5% MnO2, and 5% CuO on Al2O3 for the destruction of 2000 ppm acetic acid in water at 190° C. and 3 MPa. de Leitenburg et al reported a 14% destruction of 2000 ppm acetic acid using undoped CeO2 and up to 96% destruction of 2000 ppm acetic acid using 76% CeO2-19% ZrO2-5% MnO2 (de Leitenburg, et al., Appl. Catal B: Environ. (1996) 11, L29-L39).
Zhang and Chuang studied the effect of adding 4 wt. % Ce to 1 wt. % Pt or Pd supported on Al2O3 for oxidation of water from paper and pulp mills at 433-463K and pressures of 1.5 to 2.2 MPa and reported that 50% TOC reduction can be achieved at about an hour at 1.5 MPa and 463K (Zhang, Q. and Chuang, K. T. Applied Catalysis B: Environmental (1998) 17, 321-332).
Hocevar et al. studied the effect of catalyst preparation on phenol destruction at 432K and 7.3 bar oxygen partial pressure (total pressure in the reactor 12.1 bar) using Ce1−xCuxO2−δ catalysts, where 0.07<x<0.27 and reported samples prepared by sol-gel techniques were more active than samples prepared by co-precipitation (Hocevar, S. et al., Applied Catalysis B: Environmental (2000) 28, 113-125).
European Patent EP0514177 (Ishii) describes the use of catalysts having one component which is 90-99.95% by weight which contains 4.95 to 95 wt. % iron oxide along with the balance at least one of titanium, silicon and zirconium; and another component which is 0.05 to 10% by weight cobalt, nickel, cerium, silver, gold, platinum, palladium, rhodium, ruthenium and iridium to treat wastewater.
Mitsui describes a catalyst containing zirconium oxide (5-98 wt. %), a lanthanum oxide (2-95 wt. %) and at least one of manganese, iron, cobalt, nickel, tungsten, copper, silver, gold, platinum, palladium, rhodium, ruthenium and iridium (0.05-25 wt. %) for treating wastewater at temperatures less than or equal to 370° C. (JP4100595A2).
There is a need in the art for improved catalysts and catalytic methods for removing organic compounds from liquid media.